1. Field of the Invention
The present invention relates to a projection-type image displaying device to be used in a liquid crystal projector.
2. Related Background Art
Conventionally, a cross dichroic prism XDP has been generally used as a color combining optical element to be used in a projection-type image displaying device such as a liquid crystal projector. As shown in FIG. 15, the cross dichroic prism XDP is composed of four rectangular prisms 67, 68, 69 and 70 and dichroic layers DM1 and DM2 that have two kinds of reflection wave ranges different from each other and cross each other in the inside of the cross dichroic prism XDP.
Moreover, in case of the projection-type image displaying device using the cross dichroic prism XDP shown in FIG. 15, the incidence side of its projection lens is designed to be telecentric in order to prevent the generation of color shading owing to the cross dichroic prism XDP.
Furthermore, polarizing plates 11, 18 and 28 and polarizing plates 15, 22 and 32 are disposed on the incidence sides and the exit sides of liquid crystal panels (or liquid crystal display elements) 13, 20 and 30, respectively. And positive lenses 60, 61 and 66 are disposed on the light source sides of the liquid crystal panels 13, 20 and 30, respectively, and then the illumination light is made to be telecentric to the liquid crystal panels 13, 20 and 30, in order to prevent the generation of contrast unevenness of the liquid crystal panels 13, 20 and 30 caused by the changes of the incidence angles of illumination light to the liquid crystal panels 13, 20 and 30.
Moreover, for the improvement of contrast in such a projection-type image displaying device, Japanese Patent Application Laid-Open No. 5-323311, No. 6-148628 and No. 7-5421 propose configurations in which phase plates are disposed between polarizing plates disposed on both of the incident sides and the exit sides of liquid crystal display elements.
Moreover, Japanese Patent Application Laid-Open No. 7-13150 and No. 7-13152 propose configurations for improving the view angle characteristic of contrast in direct vision type image displaying devices using liquid crystal panels.
However, in the conventional projection-type image displaying device shown in FIG. 15, the dichroic layers DM1 and DM2 are bent at apical angles of the rectangular prisms 67 to 70 when the angles of the four rectangular prisms 67 to 70 are not correctly polished. Consequently, the conventional projection-type image displaying device has a problem such that an image projected on a screen (not shown) becomes a double image, which makes a feeling of resolution thereof remarkably worse. Moreover, when the four rectangular prisms 67 to 70 are joined, the joining requires scrupulous care lest differences in level should be occurred in order to keep the resolving power thereof on the screen good.
Furthermore, defects such as kinks, chips and the like are not allowed on the ridges (edge lines) of the rectangular prisms 67 to 70. When the width of the ridge portion is wide, a problem is also produced in which crossed portions between crossing prisms are projected on a screen as vertical stripes.
As described above, the conventional cross dichroic prism XDP is extremely hard to work a prism and to perform joining of prisms as a prism, and therefore it is difficult to reduce the manufacturing cost thereof.
On the other hand, for resolving such problems of the conventional cross dichroic prism XDP, Japanese Patent Application Laid-Open No. 10-104763 proposes a color separation prism applied to a liquid crystal projector. The color separation prism shown in FIG. 1 of the reference is composed of three prisms that have been used in a video camera or the like.
However, the shape of the color separation prism is not optimized to make the optical path length of the prism minimum. The color separation prism consequently requires an optical path length nearly twice as long as that of the cross dichroic prism.
That is, the manufacturing of the proposed color separation prism itself becomes easier in comparison with the manufacturing of the cross dichroic prism XDP because the dichroic films of the color separation prism do not cross each other in the inside of the prism. However, because the optical path length of the color separation prism is long, it is needed to set the back focal distance of the projection lens in the case of the use of the color separation prism considerably longer than that in the case of the use of the cross dichroic prism.
Consequently, the size of the projection lens in the case of the use of the color separation prism becomes large, which increases the manufacturing cost of the projection lens. Moreover, as for the performance of the projection lens especially, there is also a problem such that the chromatic aberration of magnification of the projection lens increases as the increase of the back focal distance.
Furthermore, Japanese Patent Application Laid-Open No. 5-323311 proposes a configuration in which phase difference correcting plates for correcting optical anisotropies of liquid crystal panels are disposed between incidence side polarizing plates and the liquid crystal panels for improving the contrast of the liquid crystal display elements.
In a normally white mode liquid crystal display element composed of a liquid crystal cell displaying white when the maximum voltage is applied thereto and polarizing plates disposed on the incidence side and the exit side of the liquid crystal cell, the incidence angle of illumination light at which the maximum contrast can be obtained is inclined by about three degrees from an angle perpendicular to the surface of the liquid crystal cell. Then, the main object of the configuration proposed in Japanese Patent Application Laid-Open No. 5-323311 is to compensate the phase shift produced in the liquid crystal cell by changing the phase differences of phase plates in accordance with positions.
The proposed configuration can set the liquid crystal cell to have the maximum contrast with respect to the principal ray of the illumination light that illuminates the liquid crystal cell. However, since a color image is obtained by superimposing projection images of three colors by the use of three liquid crystal panels and three projection lenses for combining three colors, there is raised a problem that the entire apparatus becomes large. Moreover, because there is only one point on the optical axis of the projection lens where the color images are completely superimposed on one another the configuration can be applied only to a rear projector or a fixed type front projector.
For resolving the constructive problems of the aforesaid configuration proposed in Japanese Patent Application Laid-Open No. 5-323311, Japanese Patent Application Laid-Open No. 6-148628 proposes to realize the improvement of contrast and the miniaturization of its size in a projection-type image displaying device which performs color combining by means of two dichroic mirrors, and in which a phase difference plate is disposed between the liquid crystal cell of a liquid crystal display element and the polarizing plate on the exit side of the liquid crystal display element so as to improve the contrast and to realize the miniaturization.
In the foregoing projection-type image displaying device, however, since the color combining is effected by the two dichroic mirrors, the back focal distance of the projection lens is needed to be twice or more as long as the length of the diagonal line of the effective display surface of the liquid crystal display element, which makes difficult to design the projection lens. Furthermore, the projection-type image displaying device has the problem of the difficulty of the complete superimposition of the projection images of three colors owing to the astigmatism caused by the dichroic mirrors and irregular deflections of their surfaces.
Moreover, the projection-type image displaying device proposed in Japanese Patent Application Laid-Open No. 6-148628 has a configuration to make the projection lens system telecentric by disposing a convex lens on the side of the projection lens of the liquid crystal display element in order to make the contrast on the image plane of a screen uniform. By employing such a configuration, the direction of the principal ray of the illumination light entering into the liquid crystal cell can be perpendicular to the surface of the liquid crystal cell anywhere on the surface. Consequently, the projection-type image displaying device does not need to change the phase of the phase plate in accordance with a position in the phase plate like the configuration proposed in Japanese Patent Application Laid-Open No. 5-323311. Then, the device has a merit of making the manufacturing of the phase plate easy.
However, the device has a problem of the decrease of contrast because light rays much enter into the liquid crystal cell from directions other than that of the angle at which the contrast becomes the maximum value in case of using a projection lens having a small f-number.
Moreover, the device proposed in Japanese Patent Application Laid-Open No. 6-148628 realizes the improvement of contrast by disposing the phase plate between the polarizing plate and the liquid crystal display element of the single-plate system liquid crystal projector having a color filer provided in the inside of the liquid crystal display element. However, because such a single-plate system utilizes light only one-third of that of three-plate system, the single-plate system has a disadvantage of the lack of sufficient brightness.
Furthermore, in the image displaying device proposed in Japanese Patent Application Laid-Open No. 7-5421, two phase plates are disposed between the polarizing plate on the incidence side and a liquid crystal cell or between the polarizing plate on the exit side and the liquid crystal cell, and at least one of the two phase plates is slightly inclined so that a slight phase difference is imparted to between light ways passing through the two phase plates to effect the phase compensation so as to improve the visual angle characteristic of the contrast and to improve the contrast on a screen surface.
Because the configuration proposed in the patent publication improves the visual angle characteristic of the contrast of the liquid crystal display element, the configuration has no possibility that the contrast is greatly decreased even if the configuration employs a projection lens having a small f-number.
However, the image displaying device proposed in the patent publication employs an optical system using two dichroic mirrors similar to those employed in the Japanese Patent Application Laid-Open No. 6-148628 as the color combining optical system thereof. The aforesaid problems in the color combination caused by the two dichroic mirrors are not resolved by the device proposed by Japanese Patent Application Laid-Open No. 7-5421.
Furthermore, in the image displaying device proposed in Japanese Patent Application Laid-Open No. 7-13150, at least one optically anisotropic element is disposed between two polarizing plates disposed on the incidence side and the exit side of a liquid crystal display element, respectively. In the image displaying device proposed in Japanese Patent Application Laid-Open No. 7-13152, two optically anisotropic elements are disposed between two polarizing plates. In the devices, the characteristics of the optically anisotropic elements and the disposition of the optical axes of the elements are restricted so that the optical anisotropies of their liquid crystal elements are corrected to correct the phase shift of polarized light that is produced in large magnitude when light enters into the liquid crystal elements obliquely. Consequently, the devices realize high contrast image displaying in wider angular ranges than those of conventional devices.
However, both the patent publications do not disclose any combination with a projection optical system.
One object of the invention is to realize a projection-type image displaying device using liquid crystals having the following features. The device is easy to manufacture. The device is capable of being miniaturized. The device can display images on a screen without any vertical stripes. And the device has a configuration in which resolving power on the screen by means of a color combining optical element is scarcely decreased.
In one aspect of the invention, a projection-type image displaying device comprises:
at least one image display element for modulating light from a light source; and
a projection optical system for projecting light from the at least one image display element;
wherein an air-reduced distance Linp between an entrance pupil of the projection optical system and a display surface of the image display element and a diagonal length Ldisp across corners of the display surface of the image display element meet the following condition:
1.5 less than Linp/Ldisp less than 4xe2x80x83xe2x80x83(1) 
In further aspect of the foregoing displaying device, the air-reduced distance Linp and the diagonal length Ldisp meet the following condition:
1.9 less than Linp/Ldisp less than 3.5xe2x80x83xe2x80x83(1A) 
In further aspect of the foregoing displaying device, an incidence side of the projection optical system is configured as a non-telecentric system.
In further aspect of the foregoing displaying device, the image display element is a liquid crystal element.
In further aspect of the foregoing displaying device, the device further comprising:
polarizing elements disposed on an incidence side and exit side of the at least one image display element; and
at least one optically anisotropic element disposed between the polarizing elements disposed on the incidence side and the exit side on an optical path on which the at least one image display element is disposed.
In further aspect of the foregoing displaying device, the at least one image display element is a plurality of image display elements for respectively modulating a plurality of light rays having wavelength ranges different from each other; and
the device further comprises a color combining optical element for combining the light rays modulated by the plural image display elements, the color combining optical element including two dichroic films therein, the dichroic films not being crossed with each other in an inside of the color combining optical element.
In further aspect of the foregoing displaying device, at least one of the two dichroic films is formed with an inclined films whose thickness or refractive index changes in a specific direction.
In further aspect of the foregoing displaying device, the color combining optical element includes at least three prisms, and an exit surface of a prism nearest to the projection optical system among the three prisms reflects light that has entered from an incidence surface of the prism nearest to the projection optical system toward one of the two dichroic films, and surface light that has entered from the side of the one of the dichroic films emerges from the exit surface.
In further aspect of the foregoing displaying device, an angle xcex81 formed by the exit surface of the prism nearest to the projection optical system in the color combining optical element and a surface of the prism which is in contact with the dichroic film, meets the following condition:
20 degrees less than xcex81 less than 35 degreesxe2x80x83xe2x80x83(2) 
In further aspect of the foregoing displaying device, the angle xcex81 meets the following condition:
23 degrees less than xcex81 less than 32 degreesxe2x80x83xe2x80x83(2A) 
In further aspect of the foregoing displaying device, an angle xcex82 formed by the exit surface of the prism nearest to the projection optical system in the color combining optical element and the dichroic film which is not in contact with the prism nearest to the projection optical system, meets the following condition:
40 degrees less than xcex82 less than 50 degreesxe2x80x83xe2x80x83(3) 
In further aspect of the foregoing displaying device, the angle xcex82 meets the following condition:
42 degrees less than xcex82 less than 48 degreesxe2x80x83xe2x80x83(3A) 
In further aspect of the foregoing displaying device, the color combining optical element includes at least four prisms, two of which are disposed between the two dichroic films; and an exit surface of a prism nearest to the projection optical system among the four prisms reflects light that has entered from an incidence surface of the prism nearest to the projection optical system toward one of the two dichroic films, and light that has entered from the side of the one of the dichroic films emerges from the exit surface.
In further aspect of the foregoing displaying device, an angle xcex81 formed by the exit surface of the prism nearest to the projection optical system in the color combining optical element and a surface of the prism which is in contact with the dichroic film meets the following condition:
20 degrees less than xcex81 less than 35 degrees. 
In further aspect of the foregoing displaying device, the angle xcex81 meets the following condition:
23 degrees less than xcex81 less than 32 degrees. 
In further aspect of the foregoing displaying device, an angle xcex82 formed by the exit surface of the prism nearest to the projection optical system in the color combining optical element and the dichroic film which is not in contact with the prism nearest to the projection optical system, meets the following condition:
40 degrees less than xcex82 less than 50 degrees. 
In further aspect of the foregoing displaying device, the angle xcex82 meets the following condition:
42 degrees less than xcex82 less than 48 degrees. 
In further aspect of the foregoing displaying device, the device further comprising a microlens array having lenses corresponding to respective pixels of the image display element, the microlens array being disposed on an incidence side of the image display element, wherein a distance between optical axes of the lenses of the microlens array are larger than a distance between pixel apertures of the image display element.
In another aspect of the invention, a projection-type image displaying device comprises:
a plurality of liquid crystal display elements for modulating a plurality of light rays having wavelength regions different from each other respectively;
a color combining optical element for combining the light rays modulated by the plural liquid crystal display elements;
a projection optical system for projecting a light ray combined by the color combining optical element;
polarizing elements disposed on an incidence side and an exit side of each of the plural liquid crystal display elements; and
at least one optically anisotropic element disposed between the polarizing elements on the incidence side and the exit side of each of the liquid crystal elements in an optical path of at least one light ray among the plural light rays;
wherein a light incidence side of the projection optical system is configured to be a non-telecentric system and an air-reduced distance Linp between an entrance pupil of the projection optical system and display surface of the liquid crystal display element and diagonal length Ldisp across corners of the display surface of the liquid crystal display element meet the following condition:
1.5 less than Linp/Ldisp less than 4. 
In further aspect of the foregoing displaying device, the air-reduced distance Linp and the diagonal length Ldisp meet the following condition:
1.9 less than Linp/Ldisp less than 3.5. 
In further aspect of the foregoing displaying device, the device further comprising a microlens array having lenses corresponding to respective pixels of the liquid crystal display element, the microlens array being disposed on the incidence side of the liquid crystal display element, wherein a distance between optical axes of the lenses of the microlens array is larger than a distance between pixel apertures of the liquid crystal display element.
In still another aspect of the invention, a image display system comprises:
The foregoing projection-type image displaying device; and
an image information supplying device for supplying image information to be projected and displayed by the projection-type image displaying device.